This invention relates in general to the field of soldering and in particular to soldering using plasmas.
Soldering involves the physical and electrical connection of components or devices using an alloy with a low melting point. An example is a lead-tin alloy with a melting point of approximately 178 degrees Celsius (C).
Most soldering processes involve the basic steps of cleaning and deoxidizing, solder reflowing, and residue cleaning. Cleaning and deoxidizing are usually accomplished by applying a flux material to remove contaminants and oxides from the surfaces to be soldered. Oxides, typically with a higher melting point than solder, can form a barrier and prevent wetting of the surfaces to be soldered if they are not removed prior to the solder reflowing. Solder reflowing joins the surfaces to be soldered when the solder is reheated beyond its melting point. Residue cleaning involves the removal of flux residue from the cleaning and deoxidizing step. Residue cleaning becomes more difficult as the physical size of components to be soldered decreases, because it is more difficult for the residue cleaning agents to penetrate small gaps between the components and the substrate.
Both wave soldering and vapor phase soldering can used to heat the solder (and the substrate on which the component(s) are to be mounted) to the melting temperature (liquidus temperature) of the solder. Both methods subject all components and the substrate to the solder melting temperature and require the use of flux. Thus, all components must be capable of withstanding the soldering temperatures and cannot be affected by the cleaning solutions used to remove the residual flux.
Hand soldering involves soldering each solder joint by hand, one at a time. This method also requires flux and cleaning the flux after soldering. In hand soldering, components are subjected to local heating which may affect the material in the component or substrate, depending on the time and temperature required to make the solder joint. The substrate and component are each subjected to a high temperature in a localized area. Because of the thermal mass of the component or area being soldered, the material in the area being soldered generally must be heated 20-40 degrees Celsius above the solder melting temperature, increasing the potential for damage.
Several fluxless soldering processes have been developed to replace the pre-cleaning step and eliminate the need for flux residue cleaning. Among these fluxless processes are sputtering, fluorinated gas plasma use, and oxygen and pure nitrogen plasma use. Sputtering, which is limited in accuracy, penetrates only short distances, and can damage the substrates and components. Fluorinated gas plasma attack certain materials such as glass, and require scrubbed exhaust to meet environmental regulations. Oxygen plasmas, while usable with gold eutectic alloys of solder, will badly oxidize tin-lead solder. Pure nitrogen plasmas do not generally provide sufficient fluxing action to cause tin-lead solder to wet the substrate or components to be soldered. Also, reflow of the solder in the fluorinated gas, oxygen, and nitrogen plasma cases is typically accomplished by conventional application of heat from a heat source separate from the plasma itself.